1. Field of the Invention
This invention relates generally to closed loop numerically controlled machining systems and more particularly to a touch probe system and gauging technique therefor.
2. Description of the Prior Art
The technology relating to automated precision machining is developing at a very rapid rate. Systems that are totally dependent on manual operations have largely given way to techniques whereby manufactured parts are made with general purpose, numerically controlled machine systems. Although cutting or other removal of material occurs automatically in such systems, numerous manual operations are still required, primarily for measuring the machined dimensions and for making cutter adjustments using an ordinary numerical control cutter offset. These manual measurements and adjustments of the cutting tool are necessary to take into account a large number of variables, such as: wear of the cutting tool; repositioning and/or replacement of the cutting tool; as well as dimensional changes of the cutting tool, of the workpiece and of the machining apparatus itself due to such factors as heating; deflection under load; etc.
By way of example, in a typical operation carried out with a numerically controlled (NC) machine tool such as a lathe, certain adjustments, e.g. tool offsets, must be manually implemented by the operator after the machine is set up for the manufacture of a particular workpiece or part. Prior to the start of machining the operator must advance the cutting tool to a tool setting surface and determine the tool position by manually measuring the space between the tool and the reference surface. This is normally done with a piece of shim material or the like, and such measurements then form the basis for manually making tool offsets. Where the lathe includes tool holding means such as a multiple tool turret, this operation must be carried out separately for each tool as well as for each of the axes of motion of the machine. Prior to making the final or finishing cut for a particular workpiece surface, the various dimensions of the semi-finished workpiece surface are measured by using a hand-held gauge. This enables the operator to determine the required offset of the cutting tool which is used for the finishing cut. After the finishing cut is made, the workpiece is again checked with the hand-held gauge in order to measure the conformance of the actual dimensions of the finished surface to the desired dimensions.
The manual operations described above are individually time consuming and take up a significant amount of the total time required to machine a particular workpiece to the desired dimensions. This serves to limit the manufacturing capacity of the machine tool. Considering present day costs of a lathe or a milling machine (machining center), any reduction of the capacity of the machine tool becomes a matter of economic significance. Further, all such manual operations further open the manufacturing process to human error.
As is generally recognized, the solution to the foregoing problems is to automate manual measurements and the manual adjustments of the cutting tool, e.g. by the use of a computer operated numerical control system. In such a system the computer may either be positioned remote from the numerical control unit, or it may be incorporated in the latter, e.g. in the form of a microcomputer. Alternatively, a computing capability may be provided remote from the numerical control unit as well as being incorporated into the latter. Instead of downloading successive blocks of data stored on tape or the like, as is the case in an ordinary NC system, a computer numerical control (CNC) system is capable of storing entire programs and calling them up in a desired sequence, editing the programs, e.g. by addition or deletion of blocks, and carrying out the computations of offsets and the like.
Although fully automatic systems have not been widely adopted at this stage of development of the precision machining field, a considerable amount of development work has been done to date, much of it limited to special purpose situations wherein a single machining operation is repetitively carried out. It is also known to mount a sensor in the form of a touch trigger probe on the bed of the machining apparatus, or on a pivotal arm that can be swung out of the way when desired. The position of the cutting tool can be calibrated against such a probe by noting the tool position when contact with the probe occurs. From the observed deviations between the programmed and the actual positions, a compensating offset may be determined and stored in the memory associated with the computer numerical control means. The offset compensates for the difference between the programmed contact position and the actual contact position.
A system and method which incorporates the features described above is disclosed in Allan R. Barlow and William A. Hunter U.S. Pat. No. 4,382,215, entitled, "System And Method Of Precision Machining", issued on May 3, 1983, and which is assigned to the assignee of the present application and incorporated herein by reference. As disclosed in this patent, a touch trigger probe known as a "Renishaw--3 Dimensional Touch Trigger Probe" is mounted in the tool holding means. The latter probe is first calibrated against datum or reference surfaces and is subsequently used to calibrate the tool sensor probe. Only then is the cutting edge of the selected tool calibrated by contact with the tool sensor probe. The initial tool offsets which are determined from the results of this operation are stored in numerical control means. After machining has taken place, the part sensor probe is again calibrated and is then used to probe the machined surface(s) of the workpiece. The information so obtained determines the final offsets required for the finishing cut. Subsequently, the finished surface may be probed to determine its conformance with the desired dimensions. Although simple in construction, the touch trigger probe must be specifically configured for a class of features to be probed. The probes themselves, which are normally purchased as commercial products from specific vendors, tend to be not only expensive but fragile and furthermore cannot reach all cuts.
Another example of touch probing is disclosed in T. Yamamato U.S. Pat. No. 4,195,250, entitled, "Automatic Measuring And Tool Position Compensating System For A Numerically Controlled Machine Tool", issued on Mar. 25, 1987. In this patent a stylus which moves under numerical control is alternately brought into contact with the workpiece. A digital type measuring system is utilized for generating a train of pulses for measurement of the amount of movement of the stylus. Pulse generation is initiated when a voltage level changes when the stylus contacts the workpiece and thus a train of pulses is started and stopped in response to the stylus contact with the workpiece providing a pulse count which is transformed into a measurement of the desired dimension. The overall system complexity is increased by the use of the apparatus employed in the system disclosed in U.S. Pat. No. 4,195,250, and therefore system reliability may be diminished with attendant adverse affects. The cost involved in its implementation is also a major factor.
Accordingly, it is an object of the present invention to provide an improvement in the gauging of machined parts.
It is a further object of the invention to provide an improvement in touch probe systems utilized in closed loop numerically controlled machining systems.
It is another object of the invention to provide a new and improved system for automatically precision machining a workpiece which utilizes apparatus that is relatively simple and economical in construction.